The invention relates to a clad sheet which is stainless on both sides and to a method for producing this clad sheet.
In many applications of steels or alloys in very corrosive mediums, such as acids, there are employed steels of the superaustenitic type or nickel-base alloys in the form of a sheet; this is the case in particular in the construction of cisterns or certain chimney flues.
The products employed contain a large amount of nickel and are very expensive. In order to reduce the cost, as it is not possible to excessively reduce the thickness for reasons relating to the properties required of the constructions realized with these sheets, one is led to the use of clad sheets. These sheets are obtained by providing a cladding of an alloy layer on a base of structural steel.
This technique presents several drawbacks:
the outer side of the base of structural steel which is not stainless may be subjected to atmospheric corrosion or a corrosion due to leakages of corrosive products when the sheet is used for making a container enclosing corrosive products; the outer side may be painted, but this is a costly operation which does not perfectly protect the surface and requires maintenance, PA1 the heat treatments required to obtain good characteristics of resistance to corrosion for alloy cladding are often incompatible with the heat treatments required for the base to obtain good mechanical properties and in particular resilience, PA1 when hot rolling, the very different rheological properties between the base and the cladding lead to unevennesses in the cladding thickness, and in order to ensure a guaranteed minimum thickness of the cladding at every point an extra thickness of cladding must be used, PA1 it is not possible to cold roll a sheet having one side of alloy and the other of structural steel owing to the very different rheological behaviour of the two materials.
When sheets having a layer of stainless steel cladding on a base of structural steel are employed for making up welded constructions, there may occur when welding the sheets a contamination of the stainless steel by the carbon or other alloy elements of the structural steel (dilution phenomenon).
The MANNESMANN patent EP 0,233,437 proposes a solution to the technical problem of compatibility between the heat treatments of the cladding and the base which consists in employing an austenoferritic stainless steel base. But this solution does not resolve the problem posed by the large rheological differences between the base and the cladding in both the hot and cold states.